Combustion at high temperature leads to the formation of NOx, or oxides of nitrogen, because of the combination of oxygen with nitrogen at the high temperature. This is a notorious pollutant and much effort is being put forth to reduce the formation of NOx.
One solution has been to premix the fuel with excess air whereby all of the combustion occurs with local high excess air. The combustion therefore occurs at relatively low temperature minimizing the formation of NOx.
A nozzle of this type is shown in U.S. Pat. No. 5,307,634 where the fuel nozzle consists of a scroll air swirler with a conical center body. The scroll swifter is made from two offset cylindrical-arc scrolls attached to two endplates. Air enters the swifter through two rectangular slots formed by the scroll offset, and exits through one endplate into the combustor by a circular hole, with the diameter substantially equal to the inscribed circle of the scrolls. Fuel is injected into the airflow at each inlet from a manifold fed linear array of orifices located on the outer scroll opposite the inner trailing edge.
This is intended to establish a uniform fuel air mixture before exit into the combustor for combustion. The portion the scroll containing the fuel entry manifold is relatively massive and cooled by the fuel itself. The trailing edge of the scroll is thin to permit the smooth flow of air thereover, and is cooled only by the hot air while it is exposed to radiation from the combustor. It is been found that because of the differential expansion between the massive cool portion of the scroll and the thin hot portion of the scroll buckling of the thin portion at the discharge end occurs. This produces variations in the flow area for the incoming air and accordingly sets forth a maldistribution of the air/fuel ratio at local areas. These areas may be local within a single nozzle or local to one of several parallel nozzles. It is desirable to maintain inlet geometry without distortion so that a uniform air/fuel mixture can be obtained.